Martin - we should definitely do this - either with the current plugin or when we move to single page checkout ... basically sagepay store the card details (So we and customer don't have to) and then you re-use the token instead of the customer re-entering their card details ... and all the customer enters is their CCV on each subsequent checkout. We'd like this in-house for pack purchases ... customers would love it on their sites.
https://www.sagepay.co.uk/support/12/36/sage-pay-token-system-understanding-the-process
NOTE: the Token system costs an additional £20 pcm. So we will need to make sure that our plugin can either detect if this is enabled via the API, or if not have an option to turn this on and off. IT would be way better not to have an option and simply assume tokens are availalble and use them if the API accepts them.